A potentially serious consequence is nutritional insufficiency which may result in impaired growth and development of infants and children. Other reported associations with giardiasis in children include malabsorption of iron, allergic reactions, inflamation of the synovial membranes of major joints, and nonprogressive retinal changes; these all requireadditional study.http://www.epa.gov/waterscience/criteria/humanhealth/microbial/giardiafs.pdf

The giardia parasite is the most common intestinal parasite worldwide, infecting an estimated 200 million people each year.

No drug can prevent giardia infection. And exposure to giardia parasites in food or municipal drinking water is unavoidable.

Each year in the United States, about 250 people in every 10,000 contract giardiasis from public drinking water.

the giardia parasite isn't just a problem in developing nations. Giardia infection (giardiasis) is one of the most common waterborne diseases in the United States. Though the parasites are often associated with backcountry streams and lakes, they also turn up in municipal water supplies, swimming pools, whirlpool spas and wells. Giardia infection, which is highly contagious, also can be transmitted through food and person-to-person contact.

Most cases of giardia infection clear up in two to six weeks without lasting effects. But some people have recurrent episodes for years or have intestinal problems long after the parasites are gone. Children who have a severe giardia infection may fail to grow normally or have learning difficulties later in life.

Lactose intolerance. Many people with giardia infection develop lactose intolerance — the inability to digest milk sugar. The problem may persist long after the infection has cleared, leading to malabsorption and weight loss in some.